1. Field of the Invention
The present invention relates to a drive method for an office-use or industrial-use ink jet printer with piezoelectric elements for ejecting ink on demand.
2. Description of the Related Art
There are thermal and piezoelectric type on-demand ink jet heads. Thermal type ink jet heads use heaters to boil a portion of ink filling the head, to generate a bubble. Ink is ejected by force of the expanding bubble. Piezoelectric type ink jet heads include a piezoelectric element that deforms a portion of an ink chamber wall, in order to apply pressure to ink in the chamber and eject an ink droplet.
Thermal heads are advantageous because they can be formed using lithography to a fine nozzle pitch of 100 xcexcm or less. However. thermal heads can only be driven at an ejection frequency of about 10 to 12 kHz during consecutive ejection. Also, only liquids with a boiling point of about 100xc2x0 C. can be used as the liquid to be ejected, which hinders broad use of thermal heads in industry.
With regard to piezoelectric type heads. piezoelectric elements deform only in small amounts. so the diaphragm in the ink chamber must have a large surface area to produce sufficient deformation for ink ejection. As a result, the nozzle pitch of piezoelectric type heads can not be formed smaller than about 140 xcexcm. However, piezoelectric type heads are well suited for high speed printing. That is, the drive frequency depends on the shape of the piezoelectric elements, so piezoelectric elements can be driven at a frequency of 20 kHz or more. Also, piezoelectric type heads are well adapted for industrial use, because in contrast to thermal type heads, they can be used to eject any type of liquid.
Conventionally, there are a variety of proposals for varying dot size ejected by ink jet heads. U.S. Pat. No. 5,495,270 discloses an ink jet apparatus capable of gray-scale printing. When the meniscus of ink in a nozzle is oscillated and a droplet ejected, the diameter of the ejected droplet will depend on the size of the center excursion (peak) in the meniscus. U.S. Pat. No. 5,495,270 discloses oscillating the meniscus to produce three different cross-sectional contours in the meniscus; (1) a contour with a single excursion, (2) a W-shaped contour. that is, with a single independent excursion and two peripheral excursions, and (3) a contour with three independent excursions. The contour (1) has the largest central excursion, contour (2) the second largest, and contour (3) the smallest. Accordingly, diameters of ejected droplets are largest to smallest in the order of contour (1) to (3). To generate meniscus oscillation of a specified contour, natural oscillation corresponding to the specified contour is used but natural oscillations corresponding to the remaining contours are not used. However, when the drive waveform is determined to contain the relevant natural oscillation, the pulse width becomes relatively long, thereby making high frequency driving difficult.
Japanese Laid-Open Patent Publication (Kokai) No. HEI-8-336970 discloses an ink jet head capable of multi-tonal printing. In order to change the volume of droplets that impinge on a recording medium, the waveform used to drive the piezoelectric element is changed to eject two ink droplets in succession, so that the two droplets merge during flight time. However, this technique is disadvantageous in that the resultant large-volume ink droplet can splash when it impinges on the recording medium, thereby staining the recording medium. Also, because a great deal of ink impinges on the recording medium at once, the ink can run so that images blur.
It is an object of the present invention to provide a method of effectively varying dot size ejected by piezoelectric type heads, which have a broad range of applications in industry.
It is another object of the present invention to provide a drive method for driving an ink jet head that enables freely changing the volume of ejected ink droplets using a drive waveform that has a pulse width of reasonable length, in order to perform high quality tonal printing without staining the recording medium.
To achieve the above and other objects, the present invention provides a drive method for an on-demand multi-nozzle ink jet head. The head includes an ink chamber filling ink therein and defined by a diaphragm and an orifice plate formed with orifices therein, a piezoelectric element which is attached to the diaphragm and deforms when a drive pulse is applied to the piezoelectric element, thereby varying pressure in the ink chamber, an ink channel for supplying ink to the ink chamber, and a common ink channel in fluid communication with the ink channel. The drive method includes the steps of:
determining a unit pulse having a pulse width, the pulse width being determining while referring to a period of Helmholtz natural oscillation;
determining an off duration paused between two successive unit pulses, the unit pulse and subsequent off duration forming a drive pulse for applying to the piezoelectric element, the off duration being equal to or less than one fourth of the pulse width of the unit pulse; and
applying a predetermined number of drive pulses in succession to the piezoelectric element when an instruction is given to increase a size of dot on a print medium.
The period of Helmholtz natural oscillation is determined based on dimensions, materials, and physical properties of the ink channel and the piezoelectric element. Preferably, the pulse width of the unit pulse is determined to be equal to the period of Helmholtz natural oscillation.
The off duration is preferably between one forth to one fifth of the pulse width of the unit pulse.
The print medium is position a predetermined distance apart from the orifice plate so that first two ink droplets ejected in response to first two drive pulses merge during flight time, and,at least one ink droplet ejected following the first two ink droplets in response to a subsequent drive pulse merges the first two ink droplets on the print medium.
The number of drive pulses for printing one combined dot may be two, three, and four. A frequency of the drive pulses can be 25 kHz at maximum.
According to the present invention, a dot of one to four times a volume of nominal dot can be printed by applying a multiple of drive pulses to the piezoelectric element in succession. This enables printing dots in desired sizes. Also, by using three unit pulses or less, printing can be performed at high frequencies of up to 25 kHz. Further, because printing is performed by merely repeating a simple pulse shape, electrical circuitry related to ink ejection can be made with a simple configuration.